In order to ensure information security, development of a wireless personal identification technique typified by SPC (Secure Private Cosm) has recently been pursued. Two-way authentication and RSSI (Received Signal Strength Indicator)-based distance detection are performed between an authentication key that is worn by a user and that has a wireless communication function and a controlled apparatus equipped with a wireless communication function, such as a personal computer, a portable phone, and a car. It is determined from the intensity of an RSSI whether or not a controlled apparatus is in an authentication area (a radius of several meters from an authentication key), and a functional lock to the controlled apparatus and operation of a warning buzzer are controlled. A function for preventing unauthorized utilization of the apparatus or mislaying of the apparatus is thereby fulfilled. When the authentication area is too narrow, the function lock or the warning buzzer is activated during the course of use of the apparatus. On the contrary, when the authentication area is too wide, security is deteriorated. Accordingly, it is desirable that the authentication area be a constant size.
However, there has been a problem of an antenna gain varying according to a distance between the authentication key and a human body, which in turn changes the size of the authentication area.
A related method for avoiding influence of a conductor, such as a human body, on an antenna is to use a loop antenna having a structure in which a loop plane is perpendicular to a conductor in order to avoid occurrence of a steep drop in gain even when the conductor gets closer to the antenna (see Patent Document 1 [FIG. 1], Patent Document 2 [FIG. 2], and Patent Document 3).    Patent Document 1: JP-A-2000-244219    Patent Document 2: JP-A-2005-109609    Patent Document 3: JP-B-3735635